Ion sources are used to generate a beam of ions, i.e. of electrically charged atoms or molecules. The various types of ion sources which are suitable for the particular requirements usually make use of a form of gas discharge to ionize neutral atoms or molecules.
The oldest, very simple ion source is the Kanalray ion source or Kanal-ray tube. In this case, a gas discharge in which the ionization takes place by electron or ion impact "burns" at a pressure of 10.sup.-1 to 1 Pa between two electrodes which carry a voltage of a few 1000 volts. This ion source, in which the electrodes are immersed in the plasma is also described as an ion source with capacitive excitation.
Another type of ion generation is achieved by means of the high-frequency ion source. In this case, the ions are generated at about 10.sup.-2 Pa by a high-frequency discharge in the MHz range which burns between two specially shaped electrodes or is generated by an external coil. The ions are drawn out of the plasma by means of a special extraction method and focused (H. Oechsner: Electron cyclotron wave resonances and power absorption effects in electrodeless low pressure H.F. plasmas with superimposed static magnetic field, Plasma Physics, 1974, Volume 16, p. 835 to 841; J. Freisinger, S. Reineck, and H. W. Loeb: The RF-Ion source RIG 10 for intense hydrogen ion beams, Journal de Physique, Colloque C7, Supplement to no. 7, Volume 40, July 1979, p. C7-477 to C7-478; I. Ogawa: Electron cyclotron resonances in a radio-frequency ion source, Nuclear Instruments and Methods 16, 1962, p. 227 to 232).
A disadvantage of many known ion sources with inductive excitation is, however, the fact that they have a substantial HF power loss. This HF power loss occurs as a result of the fact that the HF coil, which is wound round the vessel in which the plasma is located, has to be matched to the HF generator. For this purpose a matching network which matches the generator power to the load power, i.e. to the coil power, is provided between the HF generator and the HF coil (cf. e.g. German Offenlegungsschrift 2,531,812, reference numeral 40 in the figures). This matching consists in transforming the wave impedance of the coil with the plasma as load to the wave impedance of the transmitter line. In this case, a power loss of 20% to 50% of the total power delivered by the HF generator occurs in the matching circuit.
A further disadvantage of the known ion source with inductive excitation consists in the fact that the fitting of additional magnets in the vicinity of the vessel in whch the plasma is located is made more difficult because the HF coil requires a relatively large amount of space and because the magnets heat up in the magnetic field of the HF coil. Such additional magnets are required to keep the plasma away from certain points on the vessel wall or to concentrate the plasma (cf. EP-A-O, 169,744). In addition, the cooling of the coils presents problems because of the circumstance that said coils are, on the one hand, hollow and have cooling water flowing through them and, on the other hand, are at HF potential, as a result of which space-consuming potential reduction paths are required in order to bring the potential from a high value down to a low value. Since the potential reduction is achieved as a rule by lengthening the coil, an increased power loss occurs.
The construction of induction coils as hollow conductors in a current converter system and cooling with a liquid is furthermore known (German Offenlegungsschrift 2,544,275). Such liquid-cooled induction coils are also used, however, in high-frequency induction plasma burners (German Auslegeschrift 2,112,888).
Finally, a device is also known for performing a reaction between a gas and a material in an electromagnetic field, which device comprises a reaction chamber for receiving the gas and the material, an assembled coil with two coil sections linked to each other whose windings are wound in opposite directions, a high-frequency source and equipment for connecting the high-frequency source to the coil (German Offenlegungsschrift 2,245,753). In said device, the two ends of the coil are connected to each other so that they are at the same potential. In addition, one terminal of the high-frequency source is connected to a point on the coil which is located between the two ends of the coil. However, the grounded terminal of the high-frequency source is at a potential other than the ends of the coil. In the case of this device a disadvantage is also the fact that a matching network is necessary.